The present invention generally relates to an image reading apparatus, and more particularly to an image reading apparatus used in a digital copy machine having a function for copying an image formed by a slide projector.
In a copy machine in which an electrophotography process is carried out, a document is optically read, and then a photosensitive drum is exposed so that a latent image corresponding to image information obtained by reading the document is formed on the photosensitive drum. Then the latent image is developed by toner so that a toner image corresponding to the latent image is formed on the photosensitive drum. The toner image is then transferred to a recording sheet, so that a visible image corresponding to an image on the document is formed on the recording sheet. That is, the document is copied by the copy machine.
A copy machine in which an image formed by a slide projector can be copied has been proposed. In this type of copy machine, due to the shape of a light source of the slide projector, the optical property of a projector lens, the cosine-4th power rule and so on, two dimensional illuminance distribution on a plane onto which an image is projected by the slide projector is uneven. Thus, a two dimensional shading is generated on the image which is obtained by the copying of an image formed by the slide projector. Conventionally, to eliminate the two dimensional shading, the unevenness of the two dimensional illuminance distribution is corrected by use of an optical system including a lens and the like.
In addition, illuminance on a surface of the document, which is illuminated via an optical system by a light source of a scanner, varies in a main scanning direction. Thus, in this case, even if the density of the document is constant, image data output from the scanner varies in the main scanning direction. This type of variation of the image data in the main scanning direction is corrected based on a white reference. The white reference is obtained when the scanner scans a white reference plate having a predetermined density. The above correction of the variation of the image data in the main scanning direction is generally referred to as a shading correction.
However, it is difficult for the optical system to entirely correct the unevenness of the two dimensional illuminance distribution described above. Thus, the amount of light on a periphery of an image formed by the slide projector is insufficient, so that a periphery of a copied image formed on the recording sheet is dark. As the optical system has to be used for correcting the unevenness of the two dimensional illuminance distribution, the size of the copy machine and the cost thereof increase.
In addition, if correction data for each pixel with respect to a two dimensional shading based on the two dimensional illuminance distribution is stored in a memory, it is required that the memory have a large capacity.